virSecretLoad: Simplify cleanup path

virSecretLoad: Simplify cleanup path

When loading a secret value fails, the control jumps over to the
'cleanup' label where explicit call to virSecretDefFree()
happens. This is unnecessary as the corresponding variable can be
declared with g_autoptr() after which all error paths can just
return NULL instantly.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>